Electrolyte formulations form a crucial part of electrochemical and/or optoelectronic devices, and the performance of the device largely depends on the physical and chemical properties of the various components of these electrolytes.
Factors which are still impeding the technical application of many electrochemical and/or optoelectronic devices and in particular of dye or quantum dot-sensitized solar cells, concern lack of reliability caused by the volatility of organic solvents based electrolytes. It is very difficult to maintain a tight seal and retain the electrolyte in e.g. a DSC (dye-sensitized solar cell) panel, which has to withstand the temperature differences of daily day-night cycles and the concomitant thermal expansion of the electrolyte. This problem can be solved in principle by the use of ionic liquid-based electrolytes. For review “Ionic liquid electrolytes for dye-sensitized solar cells” see: M. Gorlov and L. Kloo, Dalton Trans., 2008, p. 2655-2666.
Ionic liquids or liquid salts are typically ionic species which consist of an organic cation and a generally inorganic anion usually having melting points below 373 K. Various binary ionic liquid electrolytes have recently been applied to dye-sensitized solar cells. WO 2007/093961 and WO 2009/083901 describe, thus far, the best power conversion efficiencies obtained with ionic liquid-based electrolytes for DSC containing a significant quantity of organic salts with tetracyanoborate (TCB, [(NC)4B]−) anions.
However, there continues to be a demand for new and improved electrolytes based on ionic liquids with equal or improved DSC efficiency especially at a temperature below room temperature and well above the temperature at which liquid freezing and precipitation may take place (i.e. in the range of 0° C. to 20° C.).
Some of the compounds of Formula (I), below, with imidazolium cations are disclosed in WO 2004/072089; however, the publication does not describe an electrolyte formulation comprising compounds of Formula (I) as described herein, and it does not disclose the specific utility of these compounds as components of an electrolyte formulation for the given electrochemical and/or optoelectronic devices, especially for DSC.
Organic salts are described in JP2004-175666 which includes “onium” salts with anions of formula [(CN)aX4-aB]− in which X is a halogen atom and a is an integer of 1 to 3. Trifluorocyanoborate is disclosed as a preferred anion. Electrolyte formulations containing 1-ethyl-3-methylimidazolium trifluorocyanoborate, tetraethylammonium trifluorocyanoborate or trimethyl-propylammonium trifluorocyanoborate are disclosed. There is no suggestion that compounds of Formula (I) as described below would show such improved properties over compounds containing the trifluorocyanoborate anion and the same cation.